Perched almost on the edge of the realm of the planets, the
serene blue Neptune is the last giant outpost. Another colossal world of slushy
ice and gas, Neptune has many features in common with the green planet before
it. But its hydrogen–helium atmosphere is one of constant turmoil, in contrast
to that of the pallid and unchanging Uranus. Like the other giants, Neptune is
graced by rings and satellites. Triton, its largest moon, is surprisingly
active. It has a fresh, young surface. But something is very wrong with
Neptune’s comparatively small retinue of eight moons. If astronomers are
correct, Triton is not a Neptune native – it was gravitationally captured. The
process all but destroyed Neptune’s original satellite system. And now, Triton
aside, only haphazard shards of ice and rock remain in orbit about the blue
giant.

Physical Overview

Taking almost 165 years to complete its orbit of the Sun,
Neptune is extremely distant. From Earth there is not even the faintest chance
of viewing it without binoculars or a telescope. And so, although it was
discovered in the 1840s, virtually nothing was known about this fascinating
blue gem for well over 100 years. Astronomers had to wait, twiddling their
thumbs, until 1989 before they had any real data with which to work. This was
the year in which the Voyager 2 probe made its last planetary encounter, before
heading into the depths of interstellar space.

What Voyager 2 found was an apparent twin of Uranus, at least in
terms of appearance. Neptune is slightly smaller than Uranus, by about 3 per
cent, but it has a similar colour – again due to the presence of redlight-
absorbing methane in its atmosphere. Even internally, Neptune is a lot like its
green-faced partner. It has a rocky core – perhaps a bit larger than that of
Uranus – surrounded by a vast ocean of icy slush, a thin shell of liquid
hydrogen and other materials, and a hydrogen–helium atmosphere. But there is a
surprise. While Uranus’ atmosphere is bland and featureless, Neptune’s is
banded and stormy. This is perhaps odd. The blue planet is 1.5 times further
from the Sun than its green neighbour, so its clouds ought to condense very low
down, well out of sight. The reason for the difference is that Neptune, unlike
its twin, has an unknown source of internal heat. This keeps the planet warmer
than it would otherwise be and drives the atmospheric activity so clearly
evident on the planet’s face. When Voyager 2 arrived at Neptune, the largest
atmospheric feature on display was the Great Dark Spot. Scientists immediately
likened it to a storm system, similar to the Great Red Spot on Jupiter.
Scooting around this storm, meanwhile, were a number of wispy blue-white clouds
of methane crystals. These floated some 50 kilometres above the bulk of the
atmosphere, high enough to catch the sunlight. And yet, Neptune’s great storm
has since vanished. There was no sign of it in 1994 when the Hubble Space
Telescope was swung towards the distant blue giant. Other cyclones, too, have
come and gone. Evidently Neptune’s weather patterns are not as long-lived as Jupiter’s.

Rings

Not surprisingly Neptune has rings, just like the other three
giants. And again, they are different from those we have looked at so far. As
usual, Neptune’s rings cannot compete with those of Saturn. They are very dim. Either
they are made of rocky rather than icy fragments that reflect little light, or
they are icy particles with a dark coating of organic molecules.

In total, Neptune has five rings. The most well defined, Adams
and LeVerrier, are narrow, 50 kilometres and 110 kilometres in extent
respectively. Still, these are generally wider than most of the rings of
Uranus. The particles in these two rings are most likely metre-sized boulders.
Two other rings are found between these two. One is again faint and narrow,
while the other, though still faint, is fairly broad. Called Lassell, this ring
stretches for about 4000 kilometres in radial extent. Lastly, the ring closest to
the planet, Galle, is also fairly extensive, stretching for some 2500
kilometres. As well as the boulders present in some of these rings, all contain
large amounts of dust, as found in the rings of Jupiter and Uranus. One thing
that makes Neptune’s rings stand out, though, are the so-called ring arcs. Most
of the material in the rings is evenly distributed around their circumference,
as it is in all other ring systems. But in three places, Neptune’s outermost
ring, Adams, is brighter than elsewhere. Apparently these arc-shaped segments
contain more than their fair share of particles. At first this was a mystery.
Why didn’t these arcs spread out along their orbit and repopulate the rest of
the ring? The answer, astronomers think, is that the arcs are caused by the
gravitational pull of a small moon just inside the orbit of the Adams ring.
This moon, called Galatea, is only about 160 kilometres across, with a puny
gravity. But because it is so close to the Adams ring it is able to herd the
ring fragments together and prevents them from spreading out along their orbit.
Galatea is one of eight satellites around Neptune – fewer than the other giant
planets.

Triton and Other Satellites

Like Saturn, Neptune has only one large satellite. It is called
Triton, and it is about 78 per cent the size of our own Moon. Its density
indicates a roughly 50 : 50 mixture of rock and ice. The surface is dominated
by frozen nitrogen, methane, carbon monoxide and carbon dioxide, with water ice
underneath.

Triton is an exceptional world. Perhaps most surprising of all
is that this frigid snowball is volcanically active – even though it possesses
the coldest surface temperature ever measured in the Solar System, at -235 Celsius. But its
volcanoes do not spout lava. Sunlight penetrates Triton’s transparent icy
surface and heats up underground nitrogen. As the gas boils and its pressure
rises, it escapes through cracks in the surface ice and is squirted several
kilometres above the moon’s surface into its atmosphere. This is another
surprise, that Triton should have an atmosphere. It is one of very few moons
that do. Like Saturn’s Titan, Triton holds onto its gas shroud – nitrogen with
traces of methane – because the cold gases move too sluggishly to escape the
feeble gravity. But this primitive sky is a far cry from the atmospheres of the
terrestrial planets. Some 100 000 times thinner than the air we breathe,
Triton’s gaseous envelope is almost a vacuum. Most likely, Triton’s atmosphere
derives from the surface, and the reason has to do with the moon’s orbit.
Triton encircles Neptune backwards, in a path that is highly inclined to its
planet’s equatorial plane – it is the only major satellite with such an
irregular orbit. This means that Triton’s poles, like those of Uranus, endure
long seasons when first one, and then the other – more than 80 years later – is
turned towards the Sun. This sets up a cycle on Triton in which surface gases
evaporate in the summer and help maintain the planet’s atmosphere, then freeze
out in the winter. This also explains some of Triton’s surface features. There
are few craters, probably because they are eradicated when liquid rises from
the interior and later

freezes.

While Triton is
impressive, Neptune’s other satellites are all little more than broken lumps of
ice and rock. The largest of these worldlets is Proteus. It measures 400
kilometres by 440 kilometres, and so is not a perfect sphere. Meanwhile the
smallest moons, closest to the planet, have sizes measured in just tens of
kilometres. All but one of these moonlets lie well inside the orbit of Triton
and, unlike that satellite, more or less in Neptune’s equatorial plane. But one
of the small moons chooses not to conform: Nereid. Some 340 kilometres across,
Nereid has a highly inclined orbit, like Triton, and a very elongated one – its
distance from Neptune varies by an astonishing 8 million kilometres, from 9.5
million to 1.3 million kilometres. Such an orbit is evidence, as is that of
Triton, that something very dramatic must have happened long ago in the history
of the Neptunian system.

History of the Neptunian
System

So why is Neptune’s satellite system in such disarray? Why is
Triton’s orbit so bizarre? The answer seems to be that Triton did not form
around Neptune in a disc, as did the regular satellites of the other giant
worlds. Neptune itself did grow from a disc of gas and ice just as Jupiter,
Saturn and Uranus did. It is also possible that a system of regular satellites
formed around Neptune as the planet was developing. But at some later time, Triton
– a massive, icy planetesimal in a similar orbit to Neptune’s – ventured too
close to the blue giant and got captured into its retrograde and highly
inclined orbit. During the event, Triton may well have collided with any
regular satellites that had already existed. Either they were ejected from
their orbits, or they were slingshotted into their parent world and destroyed.
Now, aside from Triton itself, only shards remain. A few still orbit in
Neptune’s equatorial plane; perhaps these are the remains of its original
satellites. Nereid, meanwhile, was almost ejected from the system – but not
quite. Evidently it did not acquire enough speed to escape entirely, and
instead settled into its very elongated elliptical meander.

As with the rings of Uranus, Neptune’s are very likely recent.
They probably did not form alongside their planet, for they would long ago have
disappeared. Again, they could be fragments of comets or small moons that were
torn apart by Neptune’s gravity.

Source :

Mark A. Garlick. The
Story Of The Solar System.University Press: Cambridge.
2002.